The present invention relates to a control device for a motor vehicle clutch of the type operated by a hydraulic cylinder. Control devices of this type simplify the operation of a motor vehicle by automatically engaging and disengaging the clutch during starting, gear changing, and stopping operations. It is preferable that the control device be suitable for use with all mass-produced vehicles. To this end, it is desirable that the control device be adapted for use with a standard clutch of the type operated by a clutch pedal. Preferably, the clutch pedal should also be retained, so that, if required, the clutch can be manually operated by the driver.
German Pat. No. 1,505,452 describes a control device of the general type described above. This control device operates as follows. When accelerating from a standstill, an electronic control circuit controls the engaging process of the clutch purely as a function of the rotary speed of the engine. This is achieved by controlling the magnitude of an electrical input signal applied to a pressure control valve as an inverse function of engine speed. The pressure control valve, in turn, controls the hydraulic pressure acting on the operating cylinder of the clutch. Accordingly, the magnitude of the electrical input, and therefore, the magnitude of the hydraulic pressure acting on the operating cylinder of the clutch, is progressively reduced as a function of increasing engine speed.
The manner in which the clutch is engaged during a gear changing operation is different from the foregoing process. During a gear changing operation, the electrical input signal is at its highest level. The magnitude of the control signal is reduced for a predetermined time period after completion of the gear-changing operation to a mean value, the level of which is a function of the position of the accelerator pedal. Not until the predetermined time period has elapsed is the electrical input, and therefore, the hydraulic pressure, reduced to the minimum value, thereby completing engagement of the clutch. In this prior art patent, engagement and disengagement of the clutch is controlled by a first control circuit when accelerating the vehicle from a standstill and by a second control circuit when changing gears.
Summarizing the foregoing, the prior art German patent utilizes two separate control circuits, each of which cause the clutch to be engaged in a different manner. Which of the two control elements are enabled (and therefore which of the two processes are used to operate the control device) is determined by the travel speed of the vehicle. Specifically, if the traveling speed lies below a specific value, for example, 15 km/h, the first control element is enabled. If the traveling speed lies above this value the second control element is enabled.
It is a primary object of the present invention to produce a control device for the automatic operation of a motor vehicle clutch which is constructed as simply as possible and which operates with a high degree of reliability under all operating conditions. The control device should also enable the duration of the slipping operation of the clutch, particularly when engaging the clutch, to be set very sensitively. If slipping operation continues for too long then there is a danger of severe wear on the clutch linings. If the clutch engages too quickly, an unpleasant torque impact, which under certain circumstances can induce vibrations in the vehicle drive, results.